1. Field of the Invention
This invention relates to an electromagnetic actuator for controlling a valve or the like.
2. Prior Art
As disclosed in U.S. Pat. No. 4,763,872 corresponding to Japanese Laid-Open (Kokai) Utility Model Application Nos. 11984/88 and 18686/88, Japanese Laid-Open (Kokai) Patent Application Nos. 98409/87 and 261782/87, and Japanese Patent Publication Nos. 7930/80 and 44540/85, a conventional electromagnetic actuator comprises three main component parts made of a magnetic material, that is, a housing and a pair of stators. These three component parts jointly constitute a magnetic circuit for a solenoid later described. More specifically, the housing has a tubular body, and the pair of stators are mounted on the opposite ends of the housing, respectively. Each of the two stators has a central tubular portion, and the tubular portions of the two stators have their one ends disposed in opposed spaced relation to each other. The solenoid of an annular shape is mounted around the tubular portions of the pair of stators, and an armature is disposed in the tubular portions of the pair of stators so as to be axially movable. When a direct current is supplied to the solenoid, a force is applied to the armature so that the armature is moved from one of the two stators toward the other.
In the above conventional electromagnetic actuator, the housing and the pair of stators are provided as three separate parts, respectively, and the pair of stators are held in face-to-face contact with the tubular housing body to form the magnetic circuit. There inevitably exists a slight or narrow space in the area of face-to-face contact between the tubular housing body and each stator, and the magnetic circuit has a relatively great magnetic resistance because of the presence of such a space. As a result, the magnetic force produced in the solenoid is small, and therefore the armature and a device operatively connected to the armature, such as a valve, could not be controlled in a stable manner.